The invention relates to toilet seat hinges, to living hinges, and to injection molding technology.
Single-material living hinges are decades old. A single-material living hinge has only one material forming both the portions being pivotally connected and the flexible connecting portion. U.S. Pat. No. 4,939,796, assigned to the assignee hereof, discloses a toilet seat hinge with a single-material living hinge. Typically using olefinic materials, these hinges can be found on a variety of products. More recent are products produced using two materials, i.e., two-material living hinges. Common to the office furniture and fenestration markets, these two-component designs have been almost entirely manufactured using the extrusion method of plastic processing. These latter applications were necessary because of the typical cracking/delamination of olefinic living hinges due to manufacturing and material variables during processing inevitably leading to breakage. This is typically found where repeated flexing has been anticipated as a due course in the life of the product.
The assignee's experience with toilet seat hinge caps incorporating a flexible hinge concept is typical of this problem. When the cap was opened repeatedly to tighten the hinge-to-china bolts it was common to have the living hinge crack or delaminate.
Two-component injection molding is newer still. Although this process has been common for the last ten years or so, the inclusion of a second material has been most commonly used for the purpose of providing a soft touch or friendly feel to a product. This has been primarily accomplished through the process of overmolding onto the structural portion of the product. A second, far less common type of two-component injection molding has been the co-injection process. In this process a skin is molded over a core of similar or dissimilar material forming a three-layer part. This process is used primarily to lower the use of prime or virgin material in the final part makeup to reduce cost. Co-injection can only be done with a two barrel specifically designed machine.